In most passenger cars the clutch is handled using only muscular force. The force is transferred from the clutch pedal inside the coupe via a direct wire connection or a hydraulic connection.
More powerful vehicles are commonly equipped with an amplifying device—a clutch servo—in order to transfer the force between the pedal and clutch. A typical clutch servo consists of a pneumatic cylinder and a control valve. It has typically a stroke length of 15 mm, a force of 3-5000N and a 60 mm additional stroke length to compensate for the wear of the clutch. A pneumatic servo requires that a compressed air system is installed in the vehicle, which generally is the case for trucks and larger buses. However, small buses and large passenger cars are normally not provided with compressed air systems.
In recent years passenger cars have been equipped with dual clutch transmissions instead of conventional automatic transmissions with planetary gearing. Such transmissions include two clutches and two drive shafts. One of the clutches is connected to the series of odd gear ratios, while the other clutch is handling the even gear ratios. When shifting gear, the next gear is ready and is quickly selected with the appropriate clutch. The gear shift occurs without power loss. The clutches are operated with electrical actuators of the solenoid type.
Such transmissions are also in demand for heavier vehicles. The actuator must then be stronger as the clutch has to transfer a larger torque. A particular problem is to develop such an electrically actuated clutch that is self-regulating. In use, the clutch plate will become worn which will offset the engagement point. In a manual transmission this is not so important, as the user will compensate for this unconsciously. The clutch stroke will also be adjusted when the vehicle is serviced. Hydraulic operated clutches may otherwise be made self-regulating. The clutch arrangement in an automatic dual clutch transmission operates within narrow tolerances, and thus it is desirable that also electrical actuators are self-regulating.